Ellipsometry is an optical measurement technique that is commonly used for a wide variety of thin film characterization applications. Fundamentals of ellipsometry, typical instrumentation, data analysis methods, and common applications may be best described in many excellent text books including Ellipsometry and Polarized Light, R. M. A. Azzam and N. M. Bashara, North Holland, 1988, Spectroscopic Ellipsometry and Reflectivty: A User's Guide, H. G. Tompkins and W. A. McGahan, Wiley-Interscience, 1999, Handbook of Ellipsometry, edited by H. G. Tompkins and E. A. Irene, William Andrew, 2006, and Spectroscopic Ellipsometry: Principles and Applications, H. Fujiwara, Wiley, 2007. Typical uses for the ellipsometry technique are measuring thin film thicknesses and optical constants. Monitoring and controlling thin films is critical for many modern technologies, and ellipsometer systems are routinely used for this purpose, both in RECD and for quality control.
Ellipsometry is a non-destructive optical technique that measures two quantities at each wavelength. These two quantities characterize the probing beam polarization state change caused by the sample surface reflection. The traditional ellipsometry expression is show below.Rp/Rs=tan(Ψ)exp(iΔ)
Rp and Rs are the complex reflectivities for p- and s-polarized light. The complex ratio Rp/Rs is parameterized by the ellipsometric parameters Ψ and Δ: the magnitude of the complex ratio is tan(Ψ), and the phase of the complex ratio is Δ. The ellipsometric Δ parameter provides extreme surface sensitivity, which enables ellipsometers to measure film thickness with sub-nanometer precision. Ellipsometry is not sensitive to the absolute intensity of the measurement beam, as it measures the ratio of p- to s-polarized reflectivity. These are distinct advantages of ellipsometry over the reflectometry technique, which only measures the intensity of the light reflected from the sample. However, ellipsometers are typically complex optical instruments which require expensive polarization optics.
An ellipsometer system generally may include a Polarization State Generator (PSG), a means for supporting the sample being measured, and a Polarization State Detector (PSD). The PSG includes a source of light, which may be monochromatic, polychromatic, or spectroscopic, and may cover any range of the electromagnetic spectrum. The PSG also includes a means for controlling, setting, and/or modulating the polarization state of the light which is emitted from the PSG. The light emitted from the PSG is reflected from or transmitted through the sample being measured. The interaction of the light from the PSG with the sample alters the polarization state of the beam, which is collected by the PSD. The PSD quantifies the polarization state of the light from the sample. As used herein, the terms Polarization State Detector, PSD, and polarimeter are considered interchangeable. Using the known polarization state set by the PSG, and the polarization state of the beam after interacting with the sample as measured by the PSD, the system may calculate ellipsometric data for the sample. The ellipsometric data for the sample may be further analyzed, using well-known methods to determine sample properties of interest, such as film thicknesses, optical constants, surface morphology, etc.
Numerous ellipsometer configurations may have been described in the prior art literature. Ellipsometer configurations differ mainly in the implementations of their PSGs and PSDs. Most modern ellipsometers are photometric instruments which use a modulated signal to improve the speed, precision, and accuracy of the measurement. Rotating element ellipsometers, which incorporate a mechanically rotating optical element in their PSG and/or PSD, have been extensively reviewed in the literature by Collins “Automatic Rotating Element Ellipsometers, Calibration, Operation, and Real Time Applications”, R. W. Collins, Review of Scientific Instruments Vol. 61, page 2029, 1990. Exemplary rotating compensator designs are also described in U.S. Pat. No. 5,872,630 and U.S. Pat. No. 6,320,657 B1. Phase modulated systems, which use a piezo-electric transducer to modulate the polarization state of the beam in the PSG and/or PSD, are also common, for example U.S. Pat. No. 5,757,671 and U.S. Pat. No. 5,956,147. However, these Patents prove limited in efficiency as requiring expensive moving parts prone to failure and costly maintenance.
Prior art ellipsometer configurations may be based on division of amplitudes polarimeter design (DOAP) which was first proposed by Azzam “Division-of-amplitude photopolarimeter (DOAP) for the simultaneous measurement of all four Stokes parameters of light”, R. M. A. Azzam, Opt. Acta Vol. 29, page 685, 1982. Features of the DOAP design may include no moving parts and four detectors to enable measurement of all four Stokes parameters which fully characterizes the polarization state of a light beam. No moving parts may potentially result in a lower cost, more robust, and higher speed polarization state detector, which may be highly advantageous for certain applications. Given these important advantages, numerous embodiments of the DOAP approach may be found in the prior art. In a DOAP polarization state detector, the light beam is divided into multiple beams by oblique reflections from beam splitters, detectors, or other optical elements. Since the intensity of the divided beams depends on the angle of the incoming beam, prior art DOAP PSD measurement errors may result if the incoming beam angle is not accurately aligned to the polarimeter.
The “classic” DOAP design first envisioned by Azzam uses a coated beam splitter to split the incoming beam into two beams, each of the two beams being further split into two beams by two Wollaston prisms, and the four resulting beam intensities detected by four detectors. Azzam's paper “Single-layer-coated beam splitters for the division-of-amplitude photopolarimeter”, R. M. A. Azzam and F. F. Sudradjat, (Applied Optics Vol. 44, No. 2, page 190, 2005) describing a method for designing an optimal coating for the beam splitter. A PSD using this design approach however, requires two expensive Wollaston prisms and an environmentally degradable custom designed and coated beam splitter. Furthermore, the coated beam splitter may be limited further by operation at a single wavelength. As the optimal beam splitter is sensitive to both the coating properties and the angle of incidence, this design is susceptible to measurement errors induced by misalignment of the incoming beam.
Azzam has proposed another method for splitting the incoming beam into multiple beams, based on diffraction from a metallic grating “Division-of-amplitude photopolarimeter based on conical diffraction from a metallic grating”, R. M. A. Azzam, (Applied Optics Vol. 31, No. 19, page 3574, 1992). However, since diffraction from a grating is highly angularly dependent, this DOAP embodiment is also highly susceptible to measurement errors induced by misalignment of the incoming beam.
Another DOAP implementation, using only four photodetectors, was proposed by Azzam in U.S. Pat. No. 4,681,450. This design provides a simplistic design with no beam splitters or optical elements required as four photodetectors simultaneously function as polarization dependent beam splitters and detectors. However, optimizing this design requires careful and time consuming orientation of angles and planes of incidences of each detector with respect to the incoming beam, which in turn makes this design highly susceptible to measurement errors induced by misalignment of the incoming beam.
A Patent to Compain and Drevillon U.S. Pat. No. 6,177,995 discloses a DOAP design similar to the “classic” Azzam DOAP, except that the '995 coated beam splitter is replaced by an uncoated prism. The uncoated prism is advantageous in that it may provide optimal polarized separation of the incoming beam in a manner that is relatively independent of both wavelength and beam angle. However, this device may still be relatively expensive to manufacture, as it uses a custom prism cut with specific angles, and two Wollaston prisms. Furthermore, while this design may be optimized to minimize measurement errors due to misalignment of the incoming beam, it suffers from lack of active error compensation.
U.S. Pat. No. 6,836,327 to Yao discloses an in-line optical polarimeter. While this invention does teach the use of polarization-selective elements arranged in an in-line orientation, Yao's device suffers since a substantial portion of the beam is transmitted through the polarimeter, and the device does not provide active correction for beam misalignment.
U.S. Pat. No. 6,177,706 also discloses a polarimeter design which uses multiple polarization sensitive interfaces to split an incoming beam into multiple beams. The '706 configuration suffers from mutual dependent and expensive polarization sensitive interfaces integrally coupled with one or more retardation layers. Similarly, the '706 Patent suffers from measurement errors due to beam misalignment.
U.S. Pat. No. 6,043,887 and U.S. Pat. No. 5,335,066 describe additional embodiments of a beam splitting polarimeter designs. These designs require that the incoming beam is split into two sub-beams, and each sub-beam is further split into two sub-beams. These designs also suffer from measurement errors due to beam misalignment.
U.S. Pat. No. 5,081,348 and U.S. Pat. No. 7,038,776 describe 4 detector polarimeters, but in these designs, the wavefront of the incoming beam is spatially split by optics. This class of polarimeter may be known as a Division of Wavefront Polarimeter (DOWP), and may suffer from errors due to changes in the beam uniformity which may affect the wavefront split.
U.S. Pat. No. 7,800,755 discloses a polarimeter having a multi-wavelength source. However, this design requires Newtonian telescope optics, and the multi-wavelength source is scanned, operatively connected to a fixed waveplate, to convert one polarization state into multiple polarization states.
U.S. Pat. No. 5,548,404 describes an additional multiple wavelength ellipsometer system. In this system, the multiple wavelength light sources are simultaneously modulated, but at different frequencies. To separate the signals from the different light sources, the system employs an expensive and cumbersome synchronous demodulation scheme.
One light source for efficient ellipsometric data measurements may include a well-known light emitting diode (LED). LED's have very long operating lifetimes (>50,000 hours), such that no light source replacement would likely be required over the lifetime of the instrument. Solid state laser diodes may also be used in the PSG. The advantages of laser diodes are a much narrower bandwidth, and higher intensities. However, compared to LED's, the operating lifetime of laser diodes may be much lower (<10,000 hours), and the output beam of a laser diode may be more difficult to collect into a uniform collimated beam. Inexpensive LED's are readily available in a variety of colors in the visible spectral range, and LED's are also available in the UV and NIR spectral ranges (though at an increased cost).
One disadvantage to using LED light sources may be the relatively large spectral bandwidth, which may exceed 30 nm Full Width Half Maximum (FWHM) for some colors of LED's. This large spectral bandwidth may corrupt the data analysis for some samples, especially for thicker films.
Another disadvantage to using LED light sources may be that their emission wavelength may shift versus both ambient temperature and drive current. This behavior may be documented in the datasheets for the devices. The wavelength shift versus temperature may be relatively small (<0.1 nm/° C.), and may be ignored for most typical applications. The wavelength shift versus drive current may be much larger (>10 nm over a typical range in drive currents) and may create variable results without accurate compensation.
An additional disadvantage to using LED light sources may include beam non-uniformities due to the LED die observed in the beam path. The die geometry in the emitting region of the LED may vary for both different LED colors and manufacturers, sometimes resulting in donut-shaped or H-shaped images at certain locations in the beam path.
U.S. Pat. No. 7,061,612 emphasizes the advantages of using a light emitting diode (LED) as light sources in a polarimeter system. This application suffers from single wavelength LED application.
U.S. Pat. No. 7,492,455 discloses a discrete polarization state spectroscopic ellipsometer system. In the '455 Patent, each light source requires an expensive polarization optic associated with it, such that when the light sources are sequentially scanned, discrete polarization states are emitted from the PSG. A single analyzer element within the PSD is limited to a partial analysis of the Stokes vector of the beam.
U.S. Pat. No. 6,034,777 discloses a method for characterizing window retardance in ellipsometer and polarimeter systems. However, the '777 patent requires a spectroscopic ellipsometric data set to simultaneously determine window characterizing and sample characterizing parameters. Another method for characterizing window retardance in ellipsometer systems is discussed in “Windows in ellipsometry measurements”, G. E. Jellison, Jr., Applied Optics Vol. 38, No. 22, page 4784, (1999). In this paper, the author suggests that is necessary to measure window characterizing properties with the windows removed from the chamber. However, this approach is inconvenient, and it may also be less accurate, as mounting the windows on the chamber may induce changes in the window characterizing properties.
Therefore a need remains for a system and related method for a multiple wavelength ellipsometer for characterizing thin film samples including efficient implementations of a multiple wavelength PSG and a no moving parts polarimeter.